1. Field of the Invention
The invention relates to a method of surface treating and compressively prestressing by peening the wall of at least one oblong cavity formed in a part, said cavity opening to the outside via a lateral mouth and having two ends capable of having frontal openings.
In particular, it is necessary to compressively prestress the wall of an axial recess of a turbomachine rotor capable of containing a blade root, the lines of contact between the axial recess and the blade root of which are highly stressed. What happens in operation is that the turbine or fan blades, immobilized in the axial recesses by various means, are subjected to considerable centrifugal forces leading to significant frictional wear at these lines of contact. This frictional wear reduces the life of the parts in operation and leads to them having to be changed regularly.
2. Summary of Prior Art
In order to improve the fatigue strength of the turbine rotor and to harden the surface of the axial recesses in the region of the lines of contact, it is known practice for the axial recesses to be peened using beads ejected from a compressed air nozzle introduced into each recess. The beads cause permanent compression of the treated surface over a shallow thickness, opposing the onset and propagation of cracks at the surface of the part.
These nozzles are able to project only microbeads of a diameter smaller than 1 mm, and typically of between 0.3 mm and 0.5 mm.
This method is necessarily lengthy because the entire surface area of the cavity is treated only by a succession of local treatments which may, furthermore, introduce unwanted local deformation and cause the incrustation of bead residue. Furthermore, in this method, the distribution of the beads, both in terms of position and in terms of speed, follows a Gaussian law.
In addition, the peening performed is light in order not to generate excessive roughness in the region of the treated zone, and this leads to a limited hardening of the surface. The problem is that since the diameter of the projected beads is small, the more intense the peening, the greater the damage caused to the surface finish.
Finally, the method, which involves numerous parameters, is difficult to control and difficult to reproduce.
The object of the invention is to propose a method for peening oblong cavities, particularly axial recesses of turbine rotors, which makes it possible to create reproducible prestresses in all the cavities, in a relatively short period of time, while at the same time allowing relatively intense peening, that is to say a greater depth of compression at the surface. This is to be achieved safely, while at the same time limiting the damage caused to, or incrustation of, the treated surface.
According to the invention, there is provided a method of surface treating and compressively prestressing by peening a wall of at least one oblong cavity formed in a part, said cavity opening to the outside via a lateral mouth and having two ends presenting frontal openings, said method comprising the steps of providing a sonotrode arranged within a sleeve, said sonotrode having a vibratory surface and ultrasonic means for vibrating said vibratory surface, and said sleeve having opposed closing-off means capable of closing off the frontal openings of said cavity, placing a plurality of beads of a predetermined diameter on said vibratory surface of said sonotrode with said sonotrode arranged in an intermediate position in which said sleeve surrounds said vibratory surface, placing said lateral mouth of said cavity facing said sonotrode, moving said sonotrode, supporting said plurality of beads, together with said sleeve substantially vertically toward a raised peening position in which said closing-off means close off said frontal openings of said cavity and in which said sonotrode is disposed facing said lateral mouth to delimit, together with said sleeve, said closing-off means and said cavity, a chamber sealed to said beads, and operating said ultrasonic means to vibrate said vibratory surface of said sonotrode whereby said beads are mobilized in said chamber to conduct ultrasonic peening of said wall of said cavity.
The method makes it possible to obtain a uniform distribution of the beads within the chamber. As the beads within the chamber travel in random directions, they strike the walls of the cavities at varying angles, and this improves the surface finish by comparison with beads projected by a nozzle in one predominant direction.
Furthermore, the entirety of the surface is subjected at the same time to the impacts of beads, and this considerably diminishes the risk of deformation of the cavity, particularly of the recess containing the blade root.
To mobilize the plurality of beads, all that is required is for at least one bead to come into contact with the vibratory surface when the sonotrode is excited. Hence the sonotrode may be arranged obliquely with respect to the vertical.
According to a preferred embodiment of the invention, the method further includes the step of moving said sonotrode substantially vertically with respect to said sleeve so that said vibratory surface closes off said mouth after the step of moving said sonotrode and said sleeve together toward said raised peening position.
As the volume in which the plurality of beads is distributed is smaller than the volume obtained when the vibratory surface remains outside the mouth, the method requires a shorter peening treatment time.
In this case, having peened the wall of a cavity, the sonotrode is moved toward its intermediate position in the sleeve, the treated cavity is replaced with another cavity to be treated, the sonotrode is moved toward its peening position and the wall of the other cavity to be treated is ultrasonically peened.
If the vibratory surface is not introduced into the mouth, having peened the wall of a cavity, the treated cavity is replaced with another cavity to be treated, and the wall of the other cavity to be treated is ultrasonically peened.
When the part comprising at least one cavity to be treated is not circular, or alternatively when the method is applied for continuously treating a succession of parts which have just one cavity, the geometry of the part is not always suited to the apparatus and it is necessary to move both the vibratory surface and the closing-off means in order to clear the space available between the steps of respectively bringing in each piece and/or cavity.
When the sleeve has to move back in order to change from one cavity to another because of the geometry of the part, having peened the wall of a cavity, the sonotrode and the sleeve are moved together toward a lowered position, the treated cavity is replaced with another cavity to be treated, the sonotrode and the sleeve are moved together toward the raised peening position, and the wall of the other cavity to be treated is ultrasonically peened.
According to an alternative embodiment, the method further includes the steps of moving said sonotrode toward its intermediate position in said sleeve after peening the wall of said cavity, moving said sonotrode and said sleeve together toward a lowered position, replacing said treated cavity with another cavity to be treated, moving said sonotrode and said sleeve together toward said raised peening position, moving said sonotrode substantialy vertically with respect to said sleeve so that said vibratory surface closes off said mouth, and ultrasonically peening the wall of said other cavity to be treated.
When the method is used to treat a circular part, for example a rotor rim, comprising a number of axial cavities formed in the periphery of said rotor rim and having wall portions which diverge from each mouth, the rotor rim is turned stepwise about its axis of rotation arranged horizontally so as bring each axial cavity in turn to face the sonotrode after a cavity has been treated.
According to a further preferred embodiment of the invention, a deflector is placed in the cavity so as to encourage the peening of the divergent wall portions of the lateral mouth.
The deflector may be of approximately triangular geometry with sides parallel to the divergent wall portions so as to reduce the effect of the peening of the zone situated between said deflector and the bottom of the cavity.
At the end of the treatment, according to a yet further preferred embodiment, the sonotrode is moved substantially vertically toward a lowered position in the sleeve, in which position the plurality of beads can be driven from the vibratory surface toward a reservoir through slots formed in the sleeve, after the cavities in the part have been peened.
Thus, the beads can easily be recovered so that they can either be reused in a subsequent treatment or be replaced.
According to an even yet further preferred embodiment, the predetermined diameter of said beads exceeds 0.8 mm.
The beads used in the method according to the invention have a larger diameter than the diameter of the beads that can be projected by a nozzle, which means that the peening can be more intense while at the same time causing less substantial damage to the surface.
The invention also provides an apparatus for use in carrying out the method, said apparatus comprising a sonotrode mounted in a sleeve equipped with closing-off means capable of closing off frontal openings of said cavity, said sonotrode being capable of projecting beads of a predetermined diameter into a chamber delimited by said wall of said cavity, said vibratory surface of said sonotrode, said sleeve and said closing-off means, means for producing ultrasonic oscillations capable of exciting said sonotrode, first means for moving said sonotrode substantially vertically, and second means for moving said sleeve substantially vertically, a first clearance, smaller than said diameter of said beads, being formed between said sonotrode and said sleeve.
According to a preferred embodiment, a second clearance, smaller than said diameter of said beads, is formed between the mouth of said cavity and said vibratory surface arranged in said mouth.
According to a further preferred embodiment, the apparatus further comprises a third clearance, smaller than the diameter of the beads, formed between one end of said sleeve and said mouth, which, when said vibratory surface is not closing off said mouth, ensures said chamber is sealed correctly.
The apparatus may comprise a number of acoustic assemblies each comprising a sonotrode and a sleeve, arranged around the rotor rim, said acoustic assemblies being capable of moving in an axial direction of the rotor rim.
The various elements of the apparatus are arranged in such a way that no bead can jam said elements that are capable of moving. Furthermore, the sonotrode and the sleeve comprising the closing-off means have a geometry tailored to the shape of the cavity to be treated. In particular, the vibratory surface is of a shape that complements the space left by mouth, and the closing-off means are formed in such a way as to close off the openings of said cavity correctly.
According to a yet further preferred embodiment, said second means for moving said sleeve are capable of moving said closing-off means and said sonotrode at the same time.
According to an even yet further preferred embodiment, said first means for moving said sonotrode and said second means for moving said sleeve can be operated simultaneously.
At the beginning of the treatment, the sleeve and the sonotrode are placed in an intermediate position in which the space generated by the closing-off means of said sleeve and the vibratory surface of said sonotrode constitutes a reservoir for the beads. Next, said sleeve and said sonotrode supporting the beads are moved jointly with the same movement. Finally, the sonotrode and/or the closing-off means can be moved individually, depending on the geometry of the cavity to be treated, until, respectively, the vibratory surface closes off the mouth of the cavity and the closing-off means close off the openings of the cavity.
According to yet another preferred embodiment, said apparatus further comprises support means for supporting at least one part that is to be treated and means for driving said support means to bring a cavity step by step over said sonotrode.
The apparatus advantageously comprises means for driving the beads from the vibratory surface toward a reservoir. These simple means make it possible to drive the beads towards the reservoir during a treatment if necessary, or at the end thereof, either to refresh the beads or to replace them.